1. Field of the Invention
The present invention relates to low-frequency hydrophones destined to be used in towed sonar linear arrays, and to sonar arrays including such hydrophones.
2. Description of the Prior Art
Towed sonar linear arrays, due to their long length, are used at low frequencies, typically between 50 Hz and 2000 Hz. They are made up of a great number of hydrophones which must have a high sensitivity, a low noise, a pressure resistance in excess of 150 bars, a low cost because of their great number, and a small size, that is dimensions shorter than 50 mm.
In the band of frequencies included between 50 Hz and 2000 Hz, the dynamic range of noise is significant. For example, sea noise decreases by 30 dB between 50 Hz and 1000 Hz. Consequently, a hydrophone is always followed by a highpass filter which eliminates the noise at the very low frequencies, and by a filter which levels sea noise and is referred to as a "whitening filter". However, the signals picked up by the hydrophone are preliminary amplified in a preamplifier located before the filters; the dynamic range of the signals being very large, this preamplifier generally saturates, and hence the filtering steps are not efficient. It is known to remedy the saturation of the preamplifier by a preliminary filtering at the hydrophone, which allows reduction of the dynamic range of the signals at the input of the preamplifier.
FIG. 1 shows a capillary hydrophone according to the prior art. This hydrophone is composed of a hollow tube 1 made of a piezoelectric ceramics such as PZT, closed at both ends by two rigid caps 2 delimiting a cavity 3. This hydrophone is pressure-balanced thanks to a capillary tube 4 attached to one of the caps and allowing the internal cavity 3 to communicate with the outside of the hydrophone to equalize the slow changes in pressure. This capillary tube 4 also serves as a mechanical filter and allows reduction of the dynamic range of the signals at the input of the preamplifier.
However, a capillary hydrophone is difficult to construct, costly and too bulky to be accommodated in towed linear arrays.
It is also known to construct very simple hydrophones with dry lacunary ceramics which are very high performance materials.
A lacunary ceramics is a composite material one phase of which is piezoelectric (high density, high permittivity and high Young modulus) and the other phase of which is gaseous (air). These ceramics, the so-called "dry lacunary ceramics", are porous and the porosity represents the gaseous volume fraction. According to a known manufacturing process, it is also possible to obtain a material with open porosity. These ceramics are characterized by a hydrostatic sensitivity increased relative to compact ceramics and having a density lower than conventional ceramics, for example a PZT ceramics has a density of about 7.3 and a hydrostatic sensitivity of about 47 picocoulomb/newton, whereas a dry lacunary ceramics with a density of about 4 has a sensitivity of about 186 picocoulombs/newton.
To construct a hydrophone, it is sufficient to deposit electrodes on a dry lacunary ceramics, to connect leads to the electrodes and to apply a coating which closes the openings of the pores. However, a weak point of this material is its brittleness, which makes impossible the applications of this type of microphone for high hydrostatic pressures at very low frequencies.